1. Field of the Invention
The present invention relates to comb filters each having comb-like frequency spectrum characteristic, and more particularly to a comb filter which is effective when used in the color signal processing circuit of video tape recorders (VTRs).
2. Description of the Prior Art
In recent years, feedback type comb filters have been used in color signal processing of VTRs. A typical example of the feedback type comb filters in the prior art as shown in FIG. 11 is disclosed in Japanese laid-open patent application No. 57-160287 (1982) and "Sony, New Systems for Video Processing ICs" by Noriyuki Yamashita et al, IEEE International Conference on Consumer Electronics, June 8, 1983. Referring to FIG. 11, input carrier color signal Cin reproduced from a magnetic tape is supplied through an input terminal 21 to an operational circuit 22. A signal C1 from the operational circuit 22 is supplied to operational circuits 25 and 26, and a 1H delay line 23 (H: horizontal scanning period). The operational circuit 26 subtracts a signal Cd from the 1H delay line 23 from the signal C1 from the operational circuit 22 to obtain an output color signal Cout which is transmitted to an output terminal 27. The operational circuit 25 adds the signal Cd from the 1H delay line 23 and the signal C1 from the operational circuit 22 to obtain a sum signal C2. The sum signal C2 is supplied through a feedback circuit 24 to the operational circuit 22.
FIG. 12 shows frequency characteristic curves of the feedback type comb filter shown in FIG. 11. When the feedback amount is 0, the comb filter shows a characteristic curve of a conventional comb filter as shown by curve a. When the feedback amount is large, it shows a characteristic curve of a feedback type comb filter as shown by curve c for example. If the feedback amount is properly adjusted, it shows characteristics between the curve a and the curve c (for example, curve b).
In the case of the curve c, chroma S/N is largely improved, but color shift or color leakage at a contour of a color image becomes large. In order to remove this defect, vertical correlation of the luminance signal is detected utilizing the fact that correlation between the luminance signal and the chroma signal is large, and the feedback amount of the chroma signal is controlled according to the vertical correlation of the luminance signal. This control is performed by a block 31 in FIG. 11. A reproduced luminance signal Y inputted to an input terminal 30 is passed through a 1H delay line 28 to obtain a delayed signal Yd , and a difference between the signal Yd and the signal Y is detected by an operational circuit 29 to obtain a signal Y1 indicative of the vertical correlation of the luminance signal. When the absolute value of the signal Y1 from the operational circuit 29 is large, the feedback amount of the feedback circuit 24 is made small so as to decrease the color leakage. When the absolute value is small, the feedback amount is made large so as to improve the chroma S/N.
In the above-mentioned configuration, however, such a premise is necessary that the correlation between the luminance signal and the chroma signal is quite large. If the correlation is small, a large color shift in the vertical direction is produced, so that the picture quality is largely deteriorated. Even when the correlation between the luminance signal and the chroma signal is large, since the configuration is similar to that of conventional comb filters, the color leakage is produced during 1H period in principle.